1. Field of the Invention
The present invention relates to flat display screens, and more specifically to so-called cathodoluminescence screens, the anode of which supports phosphor elements likely to be energized by electron bombardment. This electron bombardment can come from microtips, from layers of low extraction potential, or from a thermo-ionic source.
To simplify the present description, only color microtip screens will be considered hereafter, but it should be noted that the present invention relates, generally, to the several above-mentioned types of screens and the like, be they color or monochrome screens.
2. Discussion of the Related Art
FIG. 1 partially and schematically shows the structure of a conventional flat color microtip screen of the so-called "switched anode" type.
Such a screen is essentially formed of a cathode 1 with microtips 2 and of a grid 3 provided with holes 4 corresponding to the locations of microtips 2. Cathode 1 is placed facing a cathodoluminescent anode 5, a glass substrate 6 of which generally forms the screen surface.
The operating principle and an example of embodiment of a microtip screen are described in U.S. Pat. No 4,940,916 of the Commissariat a l'Energie Atomique.
The cathode is generally organized in columns and is formed, on a glass substrate 10, of cathode conductors organized in meshes from a conductive layer. Microtips 2 are provided on a resistive layer 11 deposited on the cathode conductors and are arranged within the meshes defined by the cathode conductors. FIG. 1 partially shows the inside of a mesh and the cathode conductors do not appear on the drawing. Cathode 1 is associated with grid 3 organized in lines. The intersection of a line of the grid and of a column of the cathode defines a pixel.
This device uses the electric field created between cathode 1 and grid 3 to extract electrons from microtips 2. These electrons are then attracted by phosphor elements 7 of anode 5 if these elements are properly biased.
Anode 5 has alternate strips of phosphor elements 7r, 7g, 7b, each corresponding to a color (Red, Green, Blue). The strips are parallel to the cathode columns and are separated from one another by an insulator 8. Phosphor elements 7 are deposited on electrodes 9, formed of corresponding strips of a transparent conductive layer such as indium and tin oxide (ITO).
In such a so-called "switched anode" screen, the sets of red, green, blue strips are, in this example, alternatively biased with respect to cathode 1, so that the electrons extracted from the microtips 2 of a pixel of the cathode/grid are alternatively directed to the phosphor elements 7 corresponding to each of the colors.
In another type of conventional microtip screen, not shown, all phosphor elements of the anode are brought to a same potential independently from their color. In this case, each column of cathode 1 is divided into three sub-columns arranged, respectively, above the strips of phosphor elements of each color. These sub-columns are addressed sequentially to bombard the phosphor elements associated with each of the colors. Each pixel is divided into three sub-pixels defined by the respective intersections of a grid line with each of the sub-columns of the cathode. Since all phosphor elements of the anode are biased independently from their color, these phosphor elements may be deposited according to a pattern defining the pixels with, for each color, an area of phosphor elements of the corresponding color defining the sub-pixel facing the corresponding cathode sub-column.
A disadvantage of conventional screens, be they switched anode screens or not, is that the microtips progressively loose their emissivity. This phenomenon can be observed by measuring the current in the cathode conductors. As a result, the brightness of the screen progressively decreases.